Snap-action temperature-responsive blade for circuit breakers



April 28, 1964 R. E. PERKINS 3,131,272

SNAP-ACTION TEMPERATURE-RESPONSIVE BLADE FOR CIRCUIT BREAKERS Filed April 15, 1961 2 Sheets-Sheet 1 F E //d "3 Z i y. i 7 J ia fi RICHARD E. PERKINS Znvenfior 59 5 by 7? z I ATTORNEY April 28, 1964 R. E. PERKINS 3,131,272

SNAP-ACTION TEMPERATURE-RESPONSIVE BLADE FOR CIRCUIT BREAKERS Filed April 13, 1961 2 Sheets-Sheet 2 ll IIIH l i i 4;: I

RICHARD E. PERKINS ATTORNEY United States Patent 3,131,272 SNAlaACTlUN TEMPERATURE-RESPONSIVE BLADE FUR CIRCUiT BREAKERS Richard E. Perkins, Ipswich, Mesa, assignor to Sylvania Electric Products Inc, a corporation of Delaware Filed Apr. 13, 1961, Ser. No. 102,730 2 Claims. (61. 206-113) This action relates to snap-action blades for circuit breakers and the like, of the kind which respond to temperature changes by deflecting with a snap action, and thereby rapidly opening or closing a contact means associated with the blades.

According to present practice, such snap blades are manufactured for use in circuit breakers, switches, and flashers by stamping them from a single sheet of laminated bimetallic material which has been spring-tempered for high resilience in directions normal to the plane of the sheet. The blade is formed with two or more longitudinally extending strips, which are interconnected at their ends. In the stamping operation or in a separate step, one of the strips is either crimped to shorten it slightly with respect to the others, or stretched to increase its length minutely. This small difference in length tends to bow the strips from the plane of the sheet relative to one another, and this resilient action is referred to herein as spring deflection. The resultant blade is stable at a given temperature in only one relative position of the strips, bowed in one direction from the undeflected plane of the blade in a configuration which depends upon the amount of crimping or stretching.

An electric contact is attached or drivingly connected to a free end of the blade, the opposite end being fixed. In operation, an increase in temperature of the blade induced by an external heat source or by an electric current passed through the blade, causes each of the bimetallic strips of the blade to deflect in the same direction from the original spring deflected position; this temperature effect is referred to herein as thermal deflection. However, the difference in length between the strips causes one of them to deflect thermally to a greater extent than the other. The blade is so designed that upon sufficient change in the relative lengths of the strips being achieved, an overcenter position is reached in which the spring-deflected bowing of the blade in one direction from the undeflected plane becomes unstable, and at least one of the strips snaps into a reverse bow, rapidly deflecting the blade in an opposite direction with respect to its original plane, and carrying the contact mounted on the free end of the blade out of engagement with a stationary contact, thus break the circuit between contacts.

The sensitivity and accuracy of the response of such a snap-action blade is entirely dependent upon the accuracy with which the strips are either crimped or stretched. Since the action of the blade depends on a minute difference in the total longitudinal deflection between the strips, it is extremely diificult to produce such blades with uniform and accurate temperature-response characteristics. The problem is made especially acute by the variety of factors which affect the amount of stretch or crimp produced by a die upon the blade, including variations in thickness of the bimetal sheet, uneven tempering of the sheet, accuracy of die configuration, and wear of the die.

It is the primary object of my invention to provide an improved temperature-responsive snap-action blade having improved sensitivity to temperature changes. It is a further object of my invention to provide an improved snap-action blade which is accurately responsive to small temperature changes, and which may be repetitively manufactured with more uniform response characteristics. It is still another object of my invention to provide an improved method for manufacturing a temperature-responsive snap-action blade for circuit breakers and the like.

Further objects and advantages of the invention will become apparent as the following description proceeds.

In general, I carry out my invention by providing a blade fabricated from at least two strips of materials interconnected at their longitudinal ends, which are selected 7 and oriented to have differing thermal deflection characdirection per unit length; the differential deflection of the strips being achieved only by their difference in overall length, which gives rise to unequal total thermal expan- SlOIlS.

In a preferred embodiment of the invention, I form one of the strips of a material having a coefficient of thermal expansion substantially equal to zero, such as beryllium copper, and form the other strip of a bimetallic material. Alternatively, one strip may be formed of a single strip of homogeneous material, and the other of a bimetallic material; or both strips may be formed of similar bimetallic materials, but oppositely oriented, with the laminant having the greater coefficient of thermal expansion facing oppositely in the two strips, with respect to the undeflected plane of the blade. In any of these arrangements the strips of the blade exhibit differing thermal deflection characteristics, since unit lengths of the strips deflect relative to one another, transversely of the longitudinal extents thereof, upon a change in temperature.

I additionally utilize spring deflection to afford a rapid and positive snap action as the blade passes through a predetermined temperature, by aifording the strips different lengths between their points of interconnection, so that they are relatively bowed. The strips are separately formed of the desired materials, and prior to the connection of their longitudinal ends, at least one of them is longitudinally compressed, for transverse deflection between its ends by spring action. According to a method of construction which comprises a feature of the invention, I secure a first end of one strip to a first end of the other by spot welding or in any other suitable manner, then insert the assembly into a fixture for holding one of the strips in a plane surface, next longitudinally compress the second strip by means of a micrometer-actuated punch to secure a predetermined spring deflection, and finally secure the second ends of the strips together by spot welding, or in any other suitable manner.

In use, the improved blade affords a positive snap action which is sensitive and accurately responsive to a predetermined temperature change, and may be uniformly reproduced in repetitively manufactured blades. This uniformity and accuracy results from reliance upon the differing thermal deflection characteristics of the two or more strips forming the blade, as distinguished from the reliance of the prior art upon a difference in overall thermal expansion arising from a relatively small difference in length of the strips.

While the specification concludes with claims clearly pointing out and distinctly claiming the subject matter which I regard as my invention, it is believed that the invention will be more clearly understood from the following detailed description of preferred embodiments thereof, referring to the accompanying drawings, in which:

FIG. 1 is a plan view of a blade according to a preferred embodiment of the invention;

FIG. 2 is a view in side elevation showing the blade in an initial position for closing contacts associated therewith;

FIG. 3 is a view similar to FIG. 2, but showing the blade in a snapped position for opening the contacts;

FIG. 4 is a View in side elevation showing a modified blade in an initial position;

FIG. is a view similar to FIG. 4 but showing the blade in a snapped position; and

FIG. 6 is a plan view illustrating an improved method for assembling the blade.

Referring to FIGS. 13, a preferred embodiment of the improved blade includes a first strip 1, which is stamped from a laminated bimetallic sheet comprising surface bonded layers of materials having dissimilar coefficients of thermal expansion. This material may be of any conventional type utilized in bimetallic switches and the like, such as a laminated sheet of steel and zinc, the latter having a substantially greater coeflicient of thermal expansion than the former. In the embodiment shown, the laminant 2 having the lower coeflicient is spaced above the laminant 3 having the higher coefficient, with the result that the strip 1 tends to bow upwardly as shown in FIG. 2 under relatively low temperature conditions. An increase in temperature, whether induced by an external heat source or by a current flowing directly through the strip, causes the laminant 3 to expand more rapidly than the laminant 2, so that the strip tends to bow downwardly toward the position shown in FIG. 3.

However, the rate of movement of the strip with respect to a change in temperature is quite slow, and for this reason a second strip 4 is provided to induce spring defiection and thus afford a snap action to the blade. In conventional practice, the second strip would be integrally attached at its longitudinal ends to the ends of the strip 1, the strips being stamped from a single sheet of the same material, and one of the strips would subsequently be stretched or crimped to afford a slight difference in effective length between the strips. Such a blade is thus subjected to spring deflection toward one of two bowed positions on opposite sides of the initial plane of the blades, depending upon the temperature prevailing in the blade. The snap action depends upon the minute difference in length between the strips, which gives rise to a difference in overall thermal expansion, although the extent and direction of thermal expansion over a unit length is the same in each strip.

According to the present invention, the second strip 4- is formed of a material selected and oriented to a different thermal deflection characteristic than the strip 1. That is to say, the strips deflect relative to one another over unit lengths thereof, transversely of their longitudinal extent, upon a change in temperature. This requirement may be met by, but is not limited to, the use of strips having different coefficients of thermal expansion. I prefer to form the strip 4 of a material having a coefficient of thermal expansion substantially equal to zero, such as beryllium copper. As shown, the strip 4 is quadrilateral in form, including longitudinal spaced-apart portions 5 and 6 interconnected at their longitudinal ends by transverse portions 7 and 8. The strip 1 is spot welded upon the upper surface of the strip 4 at 9 and 10, being initially compressed longitudinally to produce an upward bow as shown in FIG. 2.

In the application shown the strip 4 is riveted at 12 and 13, along the transverse portion 7 thereof, upon a stationary support 15. A contact 17 is brazed or otherwise suitably secured upon the opposite transverse portion 8 for engagement with a stationary contact 19 secured to the surface 15, which may be formed of an insulating material. Electrical leads (not shown) are arranged in series relation with the contacts 17 and 19, so that the blade is adapted to break the circuit by opening the contacts. As is well known in the art, the current may pass through the blade for direct heating thereof, or the blade may alternatively be heated by an external source such as an electrical resistance heater placed in the circuit, or any body whose temperature is desired to control the breaking of the circuit.

In the initial position shown in FIG. 2, the blade is at a relatively low temperature, with the strip 1 in its initial spring-deflected position. Upon an increase in temperature above a critical value established by the physical constants of the blade, the laminant 3 expands sufficiently with respect to the laminant 2 to cause an unstable or over-center relative position of the strips 1 and 4 to be reached. At this time, the thermal deflection of the strip 1 combines with spring action between the strips 1 and 4 to snap the blade rapidly toward the position of FIG. 3, thus opening the contacts 17 and 19 at a high rate of separation.

The differing thermal characteristics of the two strips combines with the spring action between them to afford a high degree of sensitivity to small changes in temperature, thus making it possible to design the blade for very accurate temperature response. Furthermore, the critical temperature of the blade at which the snap action occurs depends only upon the selection of the materials and the physical constants of the strips, as well as the degree of spring deflection applied; these factors may be controlled quite accurately, so that the blades may be repetitively manufactured with highly uniform results.

Referring to FIGS. 4 and 5, there is shown a modification of the improved blade, in which a cantilever arm 25 hearing a contact 26 is secured at an end opposite the contact to a bimetal strip 1, by spot welding, riveting, or in any other suitable manner. Parts similar to those of the embodiments of FIGS. 1-3 are similarly numbered, with prime superscripts. The construction and operation of the blade is similar to that of the first embodiment, except that the cantilever arm 25, extending from the end of the blade and being secured thereto, travels through a greater distance than does the end of the blade itself, and consequently travels at a greater speed. This expedient is desirable where an even more rapid rate of opening and closing of the contacts is necessary to minimize pitting of the contacts by their repeated opening and closing.

Referring now to FIG. 6, an improved method of assembling the blade is illustrated. I first form a strip 39 and a strip 31 of desired materials by conventional stamping operations, and then spot weld these members at first longitudinal ends thereof, as shown at 32. The assembly is placed upon a plane surface, such as the surface 34 of a chill block 35, and is held thereon as by a clamping block 37 secured to the block 35 by any suitable means (not shown). The block 37 is formed with a recess 38 such that the strip 3% will not be restrained thereby against vertical movement with respect to the surface 34. I next apply a longitudinal compression to the strip 30, causing it to deflect upwardly a predetermined distance with respect to the surface 34, in a bowed form. In the apparatus shown, this step is carried out with great accuracy by means of a micrometer-actuated punch 39, which is drivingly engaged with a micrometer feed screw 40 carried by an upright portion 41 of the block 35. Details of the micrometer device are not material to the present invention, and since such devices are well known and commercially available, no further description thereof is believed necessary. The micrometer feed screw is actuated to bring a nose surface 42 of the punch into engagement with the tip 44 of a longitudinal end of the strip 30, which is initially of sufficient length to overlap the longitudinal end of the strip 31. The micrometer screw is then actuated to compress the strip 30 a predetermined amount, depending upon the spring compression which it is desired to impart to the strip, and ranging in practice from about .001 to .010 inch. The strip is then spot welded at 45 upon the strip 31, the micrometer screw is backed off, and the block 37 is removed to release the blade. Any excess material overhanging the end of the strip 31 is then cut from the strip 30, and the blade is ready for use, having an initial configuration similar to that of the embodiment shown in FIGS. 1-3. This method of assembling the improved blade permits a desired degree of spring deflection to be accurately established.

While I have described preferred embodiments of my invention for purposes of illustration, various changes and modifications will readily occur to those skilled in the art Without departing from the true spirit and scope of the invention. It will be understood that although I prefer to form the first strip of a laminated bimetallic material, and the second strip of a material having substantially a Zero coeificient of thermal expansion, the selection of materials for these strips may be reversed. Furthermore, both strips may be formed of bimetallic materials oppositely oriented, that is, with the laminant of one strip having the greater coefiicient of thermal expansion facing oppositely, with respect to the undefiected plane of the blade, to the corresponding laminant of the other strip. Again, one strip may be bimetallic, while the other strip is formed of a single homogeneous material having a coefficient of thermal expansion not equal to zero. I intend to cover all such changes and modifications in the appended claims.

What I claim is: V

1. A snap-action thermally-responsive blade adapted for actuation of contacts in circuit breakers and the like, comprising first and second strips, one of said strips being a hollow rectangle of four legs, the other strip being fastened between the middle of one leg and the middle of the opposite leg and being of greater length between the fastenings than the other in order to subject said strips to a spring deflection force transverse to the longitudinal extents thereof, the strip of greater length being formed from a laminated bimetallic sheet of which the metals have different coefiicients of thermal expansion, and the other of said strips being straight and formed of a single material having a coeflicient of thermal expansion substantially equal to zero.

2. A snap-action thermally-responsive blade adapted for actuation of contacts in circuit breakers and the like, comprising first and second strips, one of said strips being a hollow rectangle of four legs, the other strip being fastened between the middle of one leg and the middle of the opposite leg and being of greater length between the fastenings than the other in order to subject said strips to a spring deflection force transverse to the longitudinal extents thereof, said strips being formed from laminated bimetallic sheets the materials of each of which have different coefficients of thermal expansion, the laminant having the greater coetlicient of thermal expansion in one of said strips facing oppositely, with respect to the undeflected plane of said blade, from the direction in which the corresponding material in the other of said strips faces.

References Cited in the file of this patent UNITED STATES PATENTS 2,249,582 Strobel July 15, 1941 2,656,592 Cataldo et a1. Oct. 27, 1953 2,693,027 Myer Nov. 2, 1954 2,825,779 (elleigh Mar. 4, 1958 3,037,102 Schmidinger May 29, 1962 FOREIGN PATENTS 621,637 Canada June 6, 1961 OTHER REFERENCES German application 1,065,070, printed September 10, 1959, Kl.2lc 69. 

1. A SNAP-ACTION THERMALLY-RESPONSIVE BLADE ADAPTED FOR ACTUATION OF CONTACTS IN CIRCUIT BREAKERS AND THE LIKE, COMPRISING FIRST AND SECOND STRIPS, ONE OF SAID STRIPS, BEING A HOLLOW RECTANGLE OF FOUR LEGS, THE OTHER STRIP BEING FASTENED BETWEEN THE MIDDLE OF ONE LEG AND THE MIDDLE OF THE OPPOSITE LEG AND BEING OF GREATER LENGTH BETWEEN THE FASTENINGS THAN THE OTHER IN ORDER TO SUBJECT SAID STRIPS TO A SPRING DEFLECTION FORCE TRANSVERSE TO THE LONGITUDINAL EXTENTS THEREOF, THE STRIP OF GREATER LENGTH BEING FORMED FROM A LAMINATED BIMETALLIC SHEET OF WHICH THE METALS HAVE DIFFERENT COEFFICIENTS OF THERMAL EXPANSION, AND THE OTHER OF SAID STRIPS BEING STRAIGHT AND FORMED OF A SINGLE MATERIAL HAVING A COEFFICIENT OF THERMAL EXPANSION SUBSTANTIALLY EQUAL TO ZERO. 